Erotic images prove useful in coaxing out unconscious brain activity

When your eyes are presented with erotic images in a way that keeps you from becoming aware of them, your brain can still detect and respond to the images according to your gender and sexual orientation, a team of University of Minnesota psychologists has found. The team, led by graduate student Yi Jiang and his adviser, psychology professor Sheng He, found that even when unaware of erotic images in their field of vision, research subjects shifted the focus of their visual attention according to whether they were straight males, gay males, straight women or gay/bisexual women. The researchers, who have published their findings in the Proceedings of the National Academy of Sciences online, stressed that while differences among the groups are clear, individual differences are not and so could not be used to determine a person's sexual orientation.

The purpose of the work was to uncover mechanisms by which the brain processes visual information that is not consciously perceived by the subjects. When subjects become conscious of images, the sequence of steps in brain processing becomes very complicated because neurons engage in all sorts of feedback and crosstalk--especially with emotionally charged information. The researchers were studying the flow of visual information at an earlier stage, while it is still traveling along a one-way path.

"We're trying to reveal what happens when one doesn't have a conscious visual perception. That is, how the brain processes visual information independent of consciousness," said He.

The researchers chose to generate brain activity by using erotic pictures because they promised to elicit strong responses and clear patterns in the data. But the researchers believe the mechanisms by which the brain processes such images are universal.

"This definitely doesn't just work for erotic pictures," said He. "But erotic images stand out in terms of potency to generate a response."

In the experiments, subjects were seated at a stereoscope, which allows different images to be simultaneously displayed to the left and right eyes. Each eye was presented with a square screen that was divided into two patches sitting side by side. One eye was presented with an intact picture of a nude person in one patch and the same picture scrambled in the neighboring patch. The other eye was presented with twin patches containing moving, high-contrast noise patterns similar to "snow" on a TV screen. As seen by the subjects, the screens from both eyes overlapped. The moving images had the effect of suppressing the information coming from the other eye, rendering the intact pictures invisible.

The subjects were then tested to see if their visual attention had shifted toward or away from the part of the visual field where the intact erotic image had appeared.

The strongest shift in attention toward the area where the image had been was in heterosexual men who had been shown nude female images. Those subjects also tended to be repelled by nude male images. Among heterosexual women, nude male images induced a less strong attention shift toward the image site but no significant shift in response to nude female pictures. Gay men behaved similarly to heterosexual women, and gay/bisexual women performed in between heterosexual men and women.

The divergent results among the groups of study subjects provide evidence that the subjects' brains were processing the visual information in a selective manner.

"Selective attention helps us to quickly process what is important while ignoring the irrelevant," the researchers write. "In this study, we demonstrate that information that has not entered observers' consciousness, such as [invisible] erotic pictures, can direct the distribution of spatial attention. Furthermore, invisible erotic information can either attract or repel observers' spatial attention depending on their gender and sexual orientation."

The images in the study were likely processed by the amygdala, a brain center that plays a critical role in processing emotional information, said He. But the researchers believe that the information about the images is probably destroyed at an early stage of processing by the cerebral cortex, where information from our two eyes is combined.

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The work was supported by the James S. McDonnell Foundation and the National Institutes of Health.

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